The present invention is directed to electrochemical devices and, more particularly to methods of fabricating a secondary lithium ion battery whereby the battery is conditioned (i.e., charged and discharged) at elevated temperatures prior to being packaged and sealed for immediate use or storage. The battery produces gases during the condition process which are removed.
Electrochemical cells comprise a cathode, an anode, and a polymeric matrix or separator containing an electrolyte interposed therebetween. Non-aqueous lithium electrochemical cells are discussed in U.S. Pat. Nos. 4,472,487, 4,668,595, 5,028,500, 5,441,830, 5,460,904, and 5,540,741.
The anode comprises a compatible anodic material which is any material which functions as an anode in an electrochemical cell. Such compatible anodic materials are well known in the art and include, by way of example, lithium, lithium alloys, such as alloys of lithium with aluminum, mercury, nickel, zinc, and the like, and intercalation based anodes such as carbon, WO3, and the like. The cathode comprises a compatible cathodic material which refers to any material which functions as a positive pole (cathode) in an electrochemical cell. Such compatible cathodic materials are well known in the art and include, by way of example, manganese dioxide, molybdenum trioxide, sulfides of titanium and niobium, chromium oxide, copper oxide, vanadium oxides such as V2O5, V6O13, LiV3O8 and the like. The particular compatible cathodic material employed is not critical. When the electrochemical cell is a secondary cell, then the compatible cathodic material employed is one which is capable of being recharged (e.g., LiV3O8, V6O13, MoO3, and the like).
Composite electrode refers to cathodes and anodes wherein the cathode includes materials in addition to compatible cathodic materials and the anode includes materials in addition to compatible anodic materials. Typically, the composite electrode contains a polymer which acts to bind the composite materials together and an electrolytic solvent. Composite electrodes are well known in the art. For example, a composite cathode can comprise a compatible cathodic material, a conductive material, an electrolytic solvent, an alkali salt, and a matrix forming polymer. Similarly, for example, a composite anode can comprise a compatible intercalation anodic material, an electrolyte solvent and a matrix forming polymer.
Secondary lithium ion cells and batteries employing composite electrodes are typically fabricated in the discharged state which means that the anode comprises intercalation carbon materials and the cathode comprises a suitable lithiated cathodic material, e.g., lithiated manganese oxide. Prior to use, the cell must be charged with external energy so that lithium ions from the cathodic material are intercalated into the carbon material of the anode. It has been found that during the initial charge/discharge cycles the cell generates a considerable amount of gases. These gases would be entrapped in the cell unless they are removed by a conditioning process prior to sealing the package encasing the cell.
The present invention is based, in part, on the discovery that conditioning secondary lithium ion cells at elevated temperatures reduces the time required to complete this process. Specifically, the inventive process improves the efficiency at which gas is produced and removed from the cells. In addition, this process produces cells and batteries which demonstrate improved electrochemical performance.
In one aspect, the invention is directed to a method of preparing an electrochemical cell the includes the steps of:
(a) fabricating an electrochemical cell in the discharged state which comprises (i) an anode comprising an intercalation based carbon material, an (ii) a cathode comprising a lithiated cathodic material, and (iii) a polymeric matrix or separator interposed between the anode and cathode which comprises an electrolyte solvent and salt;
(b) placing the electrochemical cell in an environment that is maintained at an elevated temperature;
(c) charging and discharging the electrochemical cell;
(d) removing gas that is generated by the electrochemical cell in step (c); and
(e) sealing the electrochemical cell.
In another aspect, the invention is directed to a method of preparing an electrochemical cell that includes the steps of:
(a) preparing an anode precursor by forming an anode film comprising an anodic material, a first polymeric binder, and a first plasticizer and thereafter removing said first plasticizer;
(b) preparing a cathode precursor by forming a cathode film comprising a cathodic material, a second polymeric binder and thereafter removing said third plasticizer;
(c) positioning a polymer electrolyte precursor between said anode film and said cathode film to form an electrochemical cell precursor and activating the electrochemical cell precursor to form an electrochemical cell;
(d) placing the electrochemical cell in an environment that is maintained at an elevated temperature;
(e) charging and discharging the electrochemical cell;
(f) removing gas that is generated from the electrochemical cell in step (e); and
(g) sealing the electrochemical cell.
In a further aspect, the invention is directed to a method of preparing an electrochemical cell that includes the steps of:
(a) forming an anode film comprising an anodic material, a polymeric matrix and a first plasticizer;
(b) forming a cathode film comprising a cathodic material, a polymeric binder and a second plasticizer;
(c) forming a polymeric or separator layer comprising a third plasticizer;
(d) interposing said polymeric or separator layer between said anode film and said cathode film;
(e) removing said plasticizers to form an electrochemical cell precursor;
(f) activation said electrochemical cell precursor to form an electrochemical cell;
(g) charging and discharging the electrochemical cell while the electrochemical cell is maintained at an elevated temperature;
(h) removing gas that is generated from the electrochemical cell during step (f); and
(i) sealing the electrochemical cell.
Preferably, the elevated temperature ranges from about 25xc2x0 C. to about 80xc2x0 C.